Heretofore, titanium oxide having high refractive index and excellent chemical stability has been used in various industrial fields.
Titanium oxide is classified based on the crystal structure thereof into rutile type, anatase type, and brookite type. However, what is actually industrially used is rutile type and anatase type.
Titanium oxide has high refractive index and high light scattering ability, and therefore, when it is in a powder state, it becomes a white powder. Titanium oxide in such a powder state is used in a large amount as a white pigment in paints, plastics, papers, inks, etc. In particular, titanium oxide having a rutile crystal structure with high thermal stability (hereinafter referred to as “rutile-type titanium oxide”) has a stable crystal structure, high thermal stability, and high light-screening ability, and therefore is used as a white pigment material for paints. Further, for example, rutile-type titanium oxide having a particle diameter of 0.1 μm or less has high transmittance with respect to visible light, but has low transmittance with respect to ultraviolet light, and therefore is often blended in sunscreen cosmetics, etc.
On the other hand, titanium oxide is an effective photocatalytically active substance, and therefore is used as a photocatalyst in various industrial fields. At present, there is no substance adopted as a photocatalyst for practical use other than titanium oxide.
In particular, titanium oxide having an anatase crystal structure (hereinafter referred to as “anatase-type titanium oxide”) has higher reduction ability than rutile-type titanium oxide and has characteristics to exhibit higher photocatalytic activity. Accordingly, in general, anatase-type titanium oxide is often used as a photocatalyst.
Anatase-type titanium oxide has a wide band gap and exhibits photocatalytic activity only when it is irradiated with ultraviolet light with a wavelength of about 380 nm or less corresponding to the band gap. For example, in the atmosphere, when anatase-type titanium oxide absorbs ultraviolet light with a wavelength of about 380 nm or less, it oxidatively decomposes an organic substance or the like located in contact with or in the vicinity of the surface of the anatase-type titanium oxide.
Further, when anatase-type titanium oxide is irradiated with the above-described ultraviolet light, it exhibits superhydrophilic activity such that the water contact angle of the surface thereof is 5° or less.
Therefore, by retaining titanium oxide on the surface of a base material, a photocatalytic function is imparted to the surface of the base material, and further, the surface becomes a hydrophilic surface. A base material in which such a photocatalytic function is imparted to the surface thereof is used in the antibacterial, deodorant, and antifouling fields, etc.
For example, if a titanium oxide coating is applied to automobile mirrors, road mirrors, etc., since the mirror surface is hydrophilic, water adhering to the surface does not turn into droplets, and dirt on the mirror surface is washed off, and thus, visibility in the rain is improved. Further, since a photocatalytic function is imparted to the surface, even if an organic impurity such as an oil is adhered thereto, the organic substance is decomposed by irradiating the surface with ultraviolet light during the day.
As described above, when titanium oxide is used as a photocatalyst in industry, it is general that the above-described titanium oxide is fixed to the surface of a base material, and a photocatalytic function is imparted to the surface of the base material. In the fixation of titanium oxide to the base material, a wet process in which a coating agent containing titanium oxide is used or a dry process in which a titanium oxide film is formed in vacuum is adopted.
What is used in a relatively wide range at present is a wet process, and as a starting material for fixing titanium oxide to a base material in a wet process, in general, a titanium oxide powder, a titanium oxide sol obtained by dispersing titanium oxide fine particles having a size of about 10 nm in water or a solvent, or a titanium compound such as a titanium alkoxide which is a precursor of titanium oxide is used. Incidentally, the above-described titanium oxide powder is used by dispersing it in a solvent, but this dispersion has a problem that when the dispersion is applied, it is easily rendered white. Therefore, in a wet process in general, a titanium oxide sol in which titanium oxide, which is fine to such an extent that light is not scattered, is dispersed without causing aggregation or a titanium compound such as a titanium alkoxide is often adopted.
The fixation of titanium oxide to a base material is performed, for example, as follows.
In the case of a titanium oxide sol, first, a titanium oxide sol is applied to a surface of a base material to which a photocatalytic function is imparted. Then, the sol is dried at a high temperature, whereby a titanium oxide film is obtained on the surface of the base material.
In the case of using a titanium compound, first, for example, a titanium alkoxide such as titanium isopropoxide is dissolved in an alcohol solvent to effect hydrolysis, whereby a titania sol in which fine particles of a hydroxide of titanium are dissolved is formed. The thus formed titania sol is applied to a surface of a base material to which a photocatalytic function is imparted, followed by firing at a temperature of, for example, about 600° C. or lower, whereby a titanium oxide film is obtained on the surface of the base material. A similar example is described in Japanese Patent Application Publication No. 2001-262008.
Incidentally, the usage examples of a titanium compound vary. For example, in Japanese Patent Application Publication No. 2011-195798 discloses an example in which titanium hydroxide obtained by hydrolyzing a halide of titanium is dissolved in an aqueous solution of an organic strong base, and further a hydroxide polymer is added thereto, and the resulting material is used.
In the case where titanium oxide is fixed to a base plate having low thermal resistance, a method in which a coating material containing titanium oxide fine particles and a curable binder (a coating agent obtained by adding a binder to a titanium oxide sol) is applied to a base material, and titanium oxide is fixed to the surface of the base material by utilizing the curing of the binder is generally used. Incidentally, an organic binder which has been conventionally used in a paint causes deterioration such as chalking in a short time due to the high oxidative ability of photocatalytic titanium oxide, and therefore, in order to design the composition of a photocatalytic coating material, an inorganic binder resistant to oxidation is used. An example of using such a binder is disclosed in, for example, Japanese Patent Application Publication No. 2003-105262.
Incidentally, as the dry process, various methods such as vapor deposition, sputtering, ion beam mixing, ion implantation, and CVD are used at present. Further, a study of a thermal spraying method in which film formation is performed in the air has been performed.
Further, recently, a thin-film production process using an anodization method utilizing an electrochemical reaction in an aqueous solution or a liquid-phase deposition (LPD) method utilizing a chemical reaction in an aqueous solution is also performed. For example, Japanese Patent No. 2785433 discloses a process in which a base material is immersed in an aqueous solution, in which ammonium fluorotitanate or titanium hydrofluoride is contained and boric acid or aluminum chloride for capturing fluorine ions is added, whereby a titanium oxide film is formed on a surface of the base material. Further, Japanese Patent Application Publication No. 10-139482 discloses a process in which a base material is immersed in an aqueous solution of titanyl sulfate, whereby a titanium oxide film is formed on a surface of the base material by hydrolysis of titanyl sulfate.
As described above, in the case where titanium oxide is fixed to a surface of a base material by a wet process, the process includes: an application step in which a titanium oxide coating agent such as a titanium oxide sol or a titanium compound such as a titanium alkoxide is applied to a surface of a base material; and a firing step in which the titanium oxide coating agent is fired along with the base material with the titanium oxide coating agent applied thereto to form a film of crystallized titanium oxide on the surface of the base material. In particular, in the case of using a titanium alkoxide, before the application step, a titania sol forming step in which fine particles of a hydroxide of titanium are dissolved, thereby forming a titania sol is needed.
That is, in a method for fixing titanium oxide as described above, the steps are complicated. Further, as described above, a firing step is needed, and therefore, the base material to which titanium oxide is applied is required to have thermal resistance.
On the other hand, in the case where it is necessary to use a base material having low thermal resistance, as described above, a coating agent obtained by adding a binder to a titanium oxide sol is applied to a base material, and titanium oxide is fixed to the surface of the base material by curing the binder. This method enables titanium oxide to be fixed to a base material having low thermal resistance, but has a problem that since a large amount of the inorganic binder is used for improving the adhesiveness of titanium oxide to a base plate, the photocatalytic activity is deteriorated.
On the other hand, in the fixation of titanium oxide to a base material by a dry process, a titanium oxide film is formed in vacuum as described above, and therefore, a large-scale film forming apparatus provided with a vacuum unit is needed. Further, in such a film forming process, generally, it is necessary to heat the base material on which a titanium oxide film is formed. Therefore, it is difficult to fix titanium oxide to a base material having low thermal resistance by a dry process. For example, in the case where a titanium oxide film is formed on a base material by a vapor deposition method, a film is formed in an undesired region of the base material, or it is difficult to control the film thickness, or the film thickness is not always uniform. Further, the process generally has a problem that a film cannot be formed on a base material having a complicated shape other than a plain plate.
In addition, in the case of using a stencil for preventing the formation of a film in an undesired region, a problem arises that the film thickness in a peripheral portion of a region in which the film is formed is increased.
The anodization method is a method in which a base material is made the anode, and the anode is oxidized by an electrochemical reaction, whereby a film is formed, and therefore, the base material on which a titanium oxide film is formed is limited to titanium or a titanium alloy.
Further, in the LPD method disclosed in Japanese Patent No. 2785433, it is necessary to use a substance which is difficult to handle and has a safety problem such as ammonium fluorotitanate or titanium hydrofluoride, and moreover, it is necessary to additionally add an agent for capturing fluorine ions, and therefore, the steps are complicated, and thus, the method has a problem from the viewpoint of practical use. On the other hand, in a film forming process utilizing hydrolysis as disclosed in Japanese Patent Application Publication No. 10-139482, it is necessary to set the temperature of a solution to a higher temperature than room temperature for accelerating the hydrolysis reaction, and thus, the uniformity of the film thickness is poor due to the effect of convection flow.